1. Field
The present description relates to methods, devices and computer program products for measurements of films of semiconductor devices and other substrates.
2. Description of Related Art
Semiconductor devices often include one or more thin conductive films. To facilitate the manufacture of such devices, it is often useful to measure the thickness of such films.
A variety of approaches have been proposed for measuring the thickness of a thin conductive film. One such approach utilizes an Eddy-Current Sensor (ECS) which operates in accordance with Faraday's law of induction and Lenz's law. FIG. 1 shows one example of such an ECS 10 which includes a coil 12 and a signal oscillator 14 such as alternating current (AC) signal source. The coil 12, driven by the oscillating signal source 14, generates an oscillating magnetic field 16 which induces circular electrical currents 20 inside a nearby conductive material 22 of a conductive film 24 of a test sample 26. The conductive film 24 is usually an electrically conductive metal. The induced eddy currents 20 in turn generate their own magnetic fields 30 which oppose the magnetic field 16 generated by the coil 12.
The interaction between the generated magnetic fields 16 and the induced magnetic fields 30 alters the complex impedance of the coil 12, which can be detected by a sensing circuit 40 (FIG. 2) connected to the coil 12. The output of the sensing circuit 40 may be interpreted by a computer or other computational device 42 to provide a useful measurement of film 24.
For example, the degree to which the complex impedance of the coil 12 is altered is typically a function of the strength of the magnetic fields 30 induced by the eddy currents 20. In turn, the strength of the induced eddy-currents 20 is a function of the electrical conductivity of the material 22 and the distance between the coil 12 and the material 22 of the film 24. When the thickness of the film 24 (as indicated by the arrows 50) is less than the penetration depth of the external magnetic field 16 (FIG. 1) at the driving frequency of the signal oscillator 14, the induced eddy-current 20 is also a function of the film material thickness 50.
Recently, an ECS sensor has been utilized as a metal film thickness measurement tool in the semiconductor industry. Thus, an ECS sensor has been used to measure the thickness of copper film deposited by an electro-plating process. However, the thickness measurement by an ECS sensor is often a function of the distance 52 between the coil 12 of the ECS sensor 10 and the film 24. This distance 52 is frequently referred to as the “lift-off” distance. Therefore, a reliable film thickness measurement may depend upon a good measurement of the lift-off distance.
One approach uses an independent displacement sensor, such as a capacitance probe or a laser triangulation sensor, to measure the lift-off distance 52 which is then relied upon by the computer 42 to compute a film thickness measurement of the film 24 based upon the outputs of the sensing circuit 40 and the independent lift-off distance measurements. This approach may have a drawback of higher complexity and cost associated with providing the independent displacement sensor. Moreover, in some implementations, there may be an additional disadvantage if the displacement sensor is mounted off axis from the ECS 10 such that the displacement sensor may not be measuring the same location on the sample 24 as the ECS 10.
Another approach attempts to compensate for an unknown lift-off distance 52 by using a sensing circuit 40 to detect both the real (resistance) and imaginary (inductive reactance) parts of the complex impedance of the coil 12 to calculate the film thickness 50 and distance 52 using an algorithm based upon iterative measurements. For example, one such sensing circuit 40 outputs both an amplitude signal and a phase signal which are a function of the complex impedance of the coil 12. The computer 42 calculates both a film thickness measurement and a lift-off measurement based upon iterative amplitude signal and phase signal outputs by the sensing circuit 40. Thus, an ECS sensor may be used as a distance or displacement sensor to measure the distance to a conductive surface.
ECS sensors are also used for other non-destructive testing of various objects. For example, ECS sensors may be used to inspect materials to detect cracks or other imperfections.